It really does help if the command you are trying to execute is actually in your pathlist which netstat.exe isn't. cmd.exe is always set in your pathlist upon Windows installation.
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Kind of an old thread but I encountered this problem tonight and wanted to post my fix, as it took some pouring through the Windrop sourceforge forums. I was having this problem on Vista64 and XP32-SP3, so I do not believe it is permissions or PATH issue ([file executable notepad.exe] will return 0 even if it is in your path anyway, I don't think Tcl takes into account env(PATH)?). Basically the problem is kind of with Tcl 8.5.
The easiest way to fix an [exec] / [open] issue that I have currently found for Windrop, is to downgrade to Windrop 1.6.17.